Pickling and rust-inhibiting bath for ferrous metals, and use of same



U it d t -"E PICKLING AND RUST-lNI-IIBITING BATH FOR FERROUS METALS, AND USE OF SAME This invention relates to a pickling-phosphatizing bath which effects the eflicient pickling of ferrous metal and at the same time phosphatizes the surface of said metal, and the useof said bath in treating ferrous metal.

The term boron-phosphate herein refers to the presence of phosphate ions and soluble boron in a picklingphosphatizing bath which has a sufficient concentration of a pickling acid from the group consisting of sulfuric acid, phosphoric acid, and admixtures thereof, to enable the bath to have a pH of not more than 1, preferably not more than 0.5. The exact chemical nature oncomposition of the boron-phosphate present in the bath cannot be defined with precision; however, boron and phosphate constituents are present in the acid bath in the form of ions, complexes, compounds, or moieties. The boron-phosphate may be separately formed and added to the acid bath as such, or may be formed in situ in the acid pickling bath.

The pickling of ferrous metal surfaces to remove rust and mill scale has been practiced for many years, particularly in steel mills which utilize hot rolling and stamping procedures. The most commonly used pickling procedure involves dipping ferrous metal articles, such 'as steel, in a hot, aqueous sulfuric acid bath comprising about 7% by volume sulfuric acid for a suflicient period to dissolve and remove the oxide scale from the surface of the article. In addition to the use of sulfuric acid in pickling baths, hydrochloric acid and phosphoric acid are sometimes used for the same purpose. The metal surface resulting from such pickling operation is somewhat roughened and is usually satisfactory for subsequent coating operations, such as painting, bonderizing, etc., but is not entirely satisfactory from the standpoint of resisting the formation of rust during storage and handling prior to such coating operations.

The use of a phosphoric acid pickling bath does not, as might be expected, produce any appreciable phosphatizing effect on the surface of ferrous metal because of the high free acid content required for pickling operations. Similarly, phosphatizing or lust-proofing procedures which utilize phosphoric acid baths are not suitable for pickling operations because of the low free acid content present in the baths used in such coating procedures.

My invention relates to a pickling bath which may be used to pickle ferrous metal surfaces, and, at the same time, produce a phosphatized surface on the metal which offers improved resistance to subsequent rusting in normally corrosive atmospheres. I have now discovered that such a pickling-phosphatizing bath may be prepared by providing an aqueous acidic pickling bath having boron-phosphate therein and which has a sufiicient concentration of a pickling acid from the group consisting of sulfuric acid, phosphoric acid, and admixtures thereof,

2,964,434 Patented Dec. 13, 0

to provide an acidic bath having a pH of not more than 1, preferably not more than 0.5.

Boron, in a soluble form, may be added to the bath as borophosphoric acid (H PO -H BO3), boric acid, borax, boron phosphate, boric acid and phosphoric acid, borax and phosphoric acid, and the like. In the event that suflicient phosphoric acid is present in the bath, as

the pickling acid, to maintain the pH of the bath at 1 or below, preferably 0.5 and below, the soluble boron compound need not be added to the bath in conjunction with phosphoric acid or as a phosphate. Regardless of how my pickling-phosphatizing bath is prepared, it is essential to my invention that boron-phosphate be present in the acidic bath and that the bath contains a sufficient concentration of free acid from the group consisting of phosphoric acid, sulfuric acid, and admixtures thereof,

to enable the bath to have a pH of not more than preferably not more than 0.5

When sulfuric acid is used as a pickling acid, the proportion of boron-phosphate that should be present in the bath may be determined by trial so as to produce a bath that effects the desired degree of rust-resistance or phosphatization on the pickled ferrous metal surfaces. It has been found that at least about 0.4% by weight boron-phosphorus compound or boron-phosphate, based on all the constituents of the bath, should be added to or present in an acidic pickling solution containing, as the pickling acid, a concentration of about 7% by volume sulfuric acid wherein the sulfuric acid has a specific gravity of 1.84. Substantially complete rust protection is obtained when the boron-phosphate or boron-phosphorus compound added to or present in a 7% by volume sulfuric acid bath is increased to about 1 to 2% by weight of all the contents of the pickling-phosphatizing bath. Although larger amounts of boron-phosphorus compound or boron-phosphate may be added to or present in the sulfuric acid pickling bath, no apparent advantage is obtained when more than 2% by weight boron-phosphate or boron-phosphorus compound is added to or present in the bath.

Also, my pickling-phosphatizing bath may be prepared by the addition of boron, in a soluble form, to a bath containing phosphoric acid as the pickling acid. For example, with a 20% by volume pickling solution of by weight phosphoric acid, the addition of 2% by weight of borophosphoric acid to the bath will give substantially complete rust-resistance to pickled steel surfaces.

The use of sulfuric acid and/or phosphoric acid, as the pickling acid, is superior to hydrochloric acid; however, when employing a 7% by weight hydrochloric acid pickling bath, 2% by weight of borophosphoric acid, based on all the constituents of the bath, will give partial rust protection. The addition of at least 5% by Weight borophosphoric acid will be required to give satisfactory rust protection for the pickled steel surface in a 7% by weight hydrochloric acid pickling bath.

For practical purposes it is preferred to use sulfuric acid as the pickling acid.

In carrying out my pickling-phosphatizing procedure, rusted or scaled steel panels were cleaned with a detergent solution to remove the grease and oil from the surface, although such cleaning is not necessary, and the panels were dipped in various acid pickling baths which had previously been heated to a temperature .of about F. The panels were allowed to remain in the pickling bath for a three-minute period or until the scale and rust were removed. The panels were then dried at the preferred temperature of about 180-210 F.; however, in cases wherein the treated steel pieces have sufiicient thickness to retain high levels of heat, artificial heating is not necessary to dry the coating. The test panels were then placed in a chamber in the corrosive atmosphere formed above asaturated solution of calcium hypochlorite. The panels were allowed to remain in the corrosive atmosphere for 4 to 24 hours, or until the surface of the control panel was completely covered with rust. The panels which had been pickled-phosphatized in an acid bath which contained boron-phosphate were checked to determine the percentage of the surface area which had become rusted.

The following table shows results of these tests wherein the baths had different compositions.

Table Approx. Percent of Example Composition ottheBaths Surface Nos. Rusted in.

Corrosion Tests Sulfuric acid (7% by vol. of 1.84 sp.g.) 75-100 Borophosphoric acid (2% by wt. of soln.) 75 Sulfuric acid (7% by vol. of 1.84 sp.g.)+0.4% 50 by wt. borophosphoric acid. a 0.8% by wt. borophosphoric acid 5 1.2% by wt. borophosphoric acid. 1 2.0% by wt. borophosphoric acid. 1 2.0% by wt. borax 100 2.0 by vol. phosphoric acid (75% H3PO4)+ 5 2.0 0 by wt. borophosphoric acid. 1.0% by vol. phosphoric acid (75% H3P04)+ 50 0.4 a by wt. borax. 1.0% by vol. phosphoric acid (75% HaPO4)-|- 3 0.8 0 by wt. borax. 1.0% by vol. phosphoric acid (75% H3PO4)+ 1 1.2 0 by Wt. borax. 2.0% by vol. phosphoric acid (75% HaPO4)+ 50 0.4% by wt. borax. 2.0% by vol. phosphoric acid (75% HaP04)+ 20 0.8% by wt. borax. 2.0% by vol. phosphoric acid (75% HaPO4)+ 1 1.6% by wt. borax. Phosphoric acid (20% by vol. of 75% HaPO4) 95 Phosphoric acid (20% by vol. of 75% H;PO4)+ 1 2.0% by wt. borophosphoric acid.

Although baths which produced panels which had about 50% of their surface free of rust may be considered as satisfactory pickling-phosphatizing solution, panels that had less than 25% rust are considered to have been treated in a distinctly superior picklingphosphatizing bath.

From the results shown in the table, it can be seen from Examples 1, 2, and that substantially no-ruste proofing effect is obtained with steel surfaces. pickled in baths wherein sulfuric acid, borophosphoric acid or phosphoric acid alone are used as the pickling acid. Example 7 shows that sulfuric acid and borax do not phosphatize ferrous metals because of the absence of a phosphate or phosphate ions in the bath. Other examples show the outstanding combination effect in the improvement in rust-resistance on the surfaces of the steel panels which were pickled and phosphatized in sulfuric acid baths which contained at least 0.4% by weight of a boronphosphorus compound.

Examples 3-6, 8-14, and 16 in the above table show that these boron-phosphate baths contained a phosphate or phosphorus to boron mole or atom ratio of 1:1 (i.e., a mole ratio of l), or a ratio exceeding this value.

In the use of phosphoric acid pickling baths, eifective phosphatizing action is obtained simply by the inclusion of a soluble boron compound dissolved in the phosphoric acid solution. If desired, boron-phosphate may be formed in situ in the acidic pickling bath as long as there is sufiicient free phosphoric acid to maintain the bath at a pH of not more than 1, preferably not more than 0.5. Example 16 illustrates the use of borophosphoric acid which is substantially equivalent to the addition of about 0.8% by weight boric. acid to the phosphoric acid pickling bath; such a low concentration of the boron 4 compound does not inhibit the pickling action of the large proportion of free phosphoric acid in the bath.

It is believed that the phosphatizing action of my new, highly acidic pickling solution is due to the formation of a complex boron-phosphate or iron-boron-phosphate coating on the freshly pickled-surface of the ferrous metal article. However, I do not wish to be held to this theory since I have been unable to detect any visible coating on such surfaces.

The illustrated examples show aqueous pickling baths to which boron, in a soluble form, is added, wherein the concentration of sulfuric acid (specific gravity of 1.84) is about 7% by volume of the bath, and the concentration of phosphoric acid in the bath is about 20% by volume of 75% by weight phosphoric acid. These concentrations of pickling acid may be varied over a wide range without causing the baths to lose their efiective pickling action. For example, pickling solutions having as low as 5% by volume of'sulfuric acid (specific gravity of 1.84) and 15% by volume of 75% by weight phosphoric acid may be used in the bath. The preferred concentrations shown above were simply selected as repsentative of the concentrations most commonly used in the industry.

The use of various acid inhibitors and wetting agents in sulfuric acid pickling baths which contained boronphosphate did not affect the rust-resistant qualities of pickled steel surfaces, although such additives may be employed in some applications wherein their own characteristics serve a useful purpose.

The foregoing detailed description has been given for clearness of understanding only, and no unnecessary limitations should be understood therefrom, as modifications will be obvious to those skilled in the art.

I claim:

1. A method of pickling and inhibiting the rusting of the surface of a ferrous metal article in a single bath comprising: immersing said article in a heated pickling and rust-inhibiting bath having a pH not greater than 1 and which contains a minor proportion of a pickling acid equivalent to not more than about 20% concentrated acid, said acid being from the group consisting of sulfuric acid, phosphoric acid, and admixtures thereof, and which contains at least about 0.4% by weight of a boron-phosphate rust inhibitor having a mole ratio of phosphorus to boron of at least about 1 to 1 and not more than about 17.3 to 1; and removing the pickled and rust-inhibited article from the bath.

2. The method of claim 1 wherein the article is immersed in a bath having a pH not greater than 0.5.

3. The method of claim 1 wherein the boron-phosphate rust inhibitor comprises a member of the group consisting of (a) borophosphoric acid, (b) phosphoric acidplus borax, and (c) phosphoric acid plus boric acid.

4. The method of claim 1 wherein said bath contains not more than about 2% by weight of said boron-phosphate rust inhibitor.

5. An acidic pickling and rust-inhibiting bath having a pH not greater than 1 and which consists essentially of water, a minor proportion of a pickling acid equivalent to not more than about 20% concentrated acid, said acid being from the group consisting of sulfuric acid, phosphoric acid, and admixtures thereof, and which contains at least about 0.4% by weight of a boron-phosphate rust inhibitor having a mole ratio of phosphorus to boron of at least about 1 to 1 and not more than about 17.3 to 1.

6. The bath of claim 5 having a pH not greater than 0.5.

7. The bath of claim 5 wherein the boron-phosphate rust inhibitor comprises a member of the group consisting of (a) borophosphoric acid, (b)v phosphoric acid plus borax, and (c)- phosphoric acid plus boric'racid.

8.. The bath of claim 5 wherein the bath contains 5 not more than about 2% by weight of said boron-phos 2,479,564 phate rust inhibitor. 2,500,673 2,559,445 References Cited in the file of this patent 2,674,552

UNITED STATES PATENTS 5 2,310,381 Zimmer Feb. 9, 1943 675,444 2,477,841 Ward Aug. 2, 1949 730,897

6 Gilbert Aug. 23, 1949 Gibson Mar. 14, 1950 Lotz July 3, 1951 Callahan et a1. Apr. 6, 1954 FOREIGN PATENTS Great Britain July 9, 1952 Great Britain June 1, 1955 UNITED STATES PATENT OFFICE CERTIFICATE. OF CORRECTION Patent No.2,.964 434 7 December 13 1960 I William B. Coleman It is hereby certified that error appearsv in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below. 1

.15 the table, second column thereof, opposite Example No. 8 for -"2-.O by vol. by vol. phosphoric acid Signed and sealed this 3rd day of October 1961,

(SEAL) Attest:

ERNEST W. SWIDER Attesting Officer DAVID L. LADD Commissioner of Patents USCOMM-D C 

1. A METHOD OF PICKLING AND INHIBITING THE RUSTING OF THE SURFACE OF A FERROUS METAL ARTICLE IN A HEATED PICKLING COMPRISING, IMMERSING SAID ARTICLE IN A HEATED PICKLING AND RUST-INHIBITING BATH HAVING A PH NOT GREATER THAN 1 AND WHICH CONTAINS A MINOR PROPORTION OF A PICKLING ACID EQUIVALENT TO NOT MORE THAN ABOUT 20% CONCENTRATED ACID, SAID ACID BEING FROM THE GROUP CONSISTING OF SULFURIC ACID, PHOSPHORIC ACID, AND ADMIXTURES THEREOF, AND WHICH CONTAINS AT LEAST ABOUT 0.4% BY WEIGHT OF A BORON-PHOSPHATE RUST INHIBITOR HAVING A MOLE RATIO OF PHOSPHORUS TO BORON OF AT LEAST ABOUT 1 TO 1 AND NOT MORE THAN ABOUT 17.3 TO 1, AND REMOVING THE PICKLED AND RUST-INHIBITED ARTICLE FROM THE BATH. 